The problems with the dialysis technique are threefold. Firstly, the dialysate is diluted into a flowing stream. Therefore, in order to have a sufficient quantity to measure, typically, a sample must be collected over ten to twenty minutes. This leads to the second problem that the technique has poor time resolution. Thirdly, the continuous removal by dialysis disturbs the concentration of the perfusing species in the region of the probe. With a view to resolving the problem of slow response time, it has been proposed to use an on-line electrode for the analysis but even so there is a delay whilst the solution flows slowly from the probe to the electrode in the electrochemical analysing equipment.
Fast response times of less than half a second may be obtained from implanted enzyme electrodes. However, the general problem with such implanted electrodes, in which an enzymic reaction is monitored by sensing changes in electrical parameters, is that the enzyme becomes inactive. It is also not easy to supply a cofactor, such as, NADH (reduced form of nicotinamide adenine dinucleotide). Furthermore, toxicity may be a barrier to use in clinical practice.